This invention relates generally to television receivers and specifically to television receivers that are capable of receiving both analog and digital signals.
The recently adopted standards for advanced television broadcasting envision the coexistence of both analog and digital television signals for a number of years. It would therefore be desirable for a single television receiver to have the capability of receiving both analog and digital signals. A solution would be to employ separate xe2x80x9cfront endsxe2x80x9d in the television receiver, one for analog signals and one for digital signals. Such a receiver would minimally require two tuners and two IF demodulators and would not be cost-effective. The solution of the invention uses one tuner and switches the IF and AGC signals between the single tuner and separate demodulators. It should be noted that the terms RF and IF are used interchangeably in the specification and the claims and the particular frequencies are not to be considered limiting of the invention. It is also recognized that many personal computer manufacturers are including (or contemplate including) additional circuitry in their computers to enable the computer to function as a television receiver. Accordingly, the invention should not be considered as limited to television receivers.
A potential solution to the above problem is to incorporate an RF splitter between the tuner and the demodulators to permit supplying both demodulators with the IF signal. The signal loss in the splitter would, however, appear to seriously degrade the receiver noise performance. Another potential solution is to amplify the IF signal before splitting it. This technique could seriously degrade the distortion characteristics of the digital receiver in the presence of strong adjacent channel analog signals.
The invention in both the present and prior applications is directed to a low cost switching system for supplying the IF output from a single tuner to separate analog and digital demodulators. The inventive system in a first aspect is covered in the prior application and incorporates an RF relay to switch the IF signal between the analog and digital demodulators and to couple the AGC signal back to the tuner. In another aspect of that invention, the switching is minimized by multiplexing the very low frequency AGC signal with the high frequency (45 MHz) IF signal. The multiplexing is accomplished by means of separate high frequency and low frequency paths between the tuner and the RF switch, between the RF switch and the analog demodulator and between the RF switch and the digital demodulator.
The present invention precludes the possibility of interference being generated by interaction of the oscillators in the demodulators, both of which are active. It also permits use with equipment from different manufacturers that may operate at different voltage levels.
A principal; object of the invention is to provide an improved low cost method of processing digital and analog television signals using a single tuner.
Another object of the invention is to provide a single-tuner television receiver that operates to receive analog and digital signals with minimal oscillator interference.